Biocompatibility is a critical concern for medical devices that are designed to be implanted in vivo. Biocompatibility is necessary to avoid adverse reactions in the subject, and to avoid device failure as a result of exposure to the corrosive saline body fluids and other substances in the tissue surrounding the implant. Where an implanted device includes one or more components that are not, themselves, biocompatible, it is known to provide hermetic sealing of such devices with a chemically inert coating to achieve biocompatibility, i.e., in order to avoid adverse reactions and device degradation. Many such implantable devices are intended to remain in place over long periods of time, imposing a long life requirement on the manner of hermetic sealing.
Miniature implantable medical devices commonly include microelectronic components, such as integrated circuit chips fabricated on silicon substrates. Ion beam assisted deposition (“IBAD”) of alumina, often referred to an aluminum oxide (Al2O3), has been proposed for hermetically sealing such devices. Alumina has good biocompatibility, and IBAD is a useful technique for depositing dense, adherent, defect-free conformal thin films. The use of IBAD to deposit alumina on implantable medical devices is described in U.S. Pat. No. 6,844,023, entitled “Alumina Insulation For Coating Implantable Components And Other Microminiature Devices,” the disclosure of which is incorporated by reference.
Most implantable microelectronic devices require means for connecting to the devices for purposes of supplying power to the device or for routing electrical signals to or from the device. Such devices include, for example, stimulators which operate by providing current to the surrounding tissue, and sensors which measure chemical or electrical properties of the surrounding tissue. When an insulator, such as alumina, is used as a coating on a device to provide hermetic sealing, a conductive path through the alumina to an external contact is typically required. For implantable devices fabricated on silicon using standard silicon processing technology, the contact pads on the device are normally copper or aluminum, neither of which is biocompatible. Thus, semiconductor device contact pads cannot simply be left exposed by patterning the surrounding alumina layer.
A prior art structure addressing the need to provide means for connecting to a sealed, implantable electronic device is disclosed in U.S. Pat. No. 6,516,808, entitled “Hermetic Feedthrough For An Implantable Device,” the disclosure of which is also incorporated by reference. The '808 patent depicts several embodiments of “hermetic” electrical feedthrough structures. Thus, the embodiment of FIGS. 5A and 5B of the patent show simple via structures, while the embodiments of FIGS. 6A, 6B and 7 show “serpentine” feedthrough structure which are said to provide greater “hermeticity.” Implicit in the '808 patent's discussion of the serpentine feedthrough structures, and as confirmed by the inventors hereof, is the fact that the prior art simple via feedthrough structures are not adequately hermetic, particularly in applications where they will remain in vivo for a lengthy period. While use of a serpentine structure may overcome this inadequacy, such structures are generally more difficult to fabricate and, in some instances, consume valuable “real estate” on the surface of the device.
Another approach to providing a hermetic electrical path through a conformal electrically insulating film is described in co-assigned U.S. Pat. No. 6,858,220, the disclosure of which is incorporated by reference. The '220 patent describes extremely thin (e.g., 40 nm) ultra-nanocrystalline diamond coatings wherein an electric path through the film is created by selective ion implantation. Unfortunately, this solution has limited applicability to extremely thin films that can be rendered suitably conductive by ion implantation.
Accordingly, a structure which provides better hermetic sealing of a feedthrough between an implantable microelectronic device and the surface of an encapsulating insulator is needed.